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This is probably how fish evolved to walk on land

Researchers are using a living fish, called Polypterus, to help show what might have happened when fish first tried to walk out of the water.

Polypterus is an African fish that can breathe air, “walk” on land, and looks much like those ancient fishes that evolved into tetrapods.

About 400 million years ago, a group of fish began exploring land and evolved into tetrapods—today’s amphibians, reptiles, birds, and mammals. But just how these ancient fish used their fishy bodies and fins in a terrestrial environment and what evolutionary processes were at play remain scientific mysteries.

The team of researchers raised juvenile Polypterus on land for nearly a year, with the aim of revealing how these “terrestrialized” fish looked and moved differently.

“Stressful environmental conditions can often reveal otherwise cryptic anatomical and behavioral variation, a form of developmental plasticity,” says Emily Standen, a former McGill University postdoctoral student who led the project, now at the University of Ottawa.

“We wanted to use this mechanism to see what new anatomies and behaviors we could trigger in these fish and see if they match what we know of the fossil record.”

On their fins

As reported in Nature, the fish showed significant anatomical and behavioral changes. The terrestrialized fish walked more effectively by placing their fins closer to their bodies, lifted their heads higher, and kept their fins from slipping as much as fish that were raised in water.

“Anatomically, their pectoral skeleton changed to become more elongate with stronger attachments across their chest, possibly to increase support during walking, and a reduced contact with the skull to potentially allow greater head/neck motion,” says Trina Du, a McGill PhD student and study collaborator.

“Because many of the anatomical changes mirror the fossil record, we can hypothesize that the behavioral changes we see also reflect what may have occurred when fossil fish first walked with their fins on land,” says Hans Larsson, Canada Research Chair in Macroevolution at McGill.

The terrestrialized Polypterus experiment is unique and provides new ideas for how fossil fishes may have used their fins in a terrestrial environment and what evolutionary processes were at play.

Larsson adds, “This is the first example we know of that demonstrates developmental plasticity may have facilitated a large-scale evolutionary transition, by first accessing new anatomies and behaviors that could later be genetically fixed by natural selection”.

The Canada Research Chairs Program, Natural Sciences and Engineering Research Council of Canada (NSERC), and Tomlinson Postdoctoral fellowship supported the work.

Source: McGill University

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